Emerald Lake is one of the most beautiful lakes in Canadian Rockies (probably the third most famous after Lake Louise and Moraine Lake), so it is definitely a touristy place, but it's a must see in Yoho NP. Even better if you can make it for sunrise or sunset.

There is a loop trail around the lake which is an easy 5 km. You can also rent a canoe at the boat house which is such a bucket list (though expensive) thing to do. The lake is also a popular XC ski destination in the winter.